Field of the Invention
This application claims the benefit of Japanese Patent Application No. 2016-106305 filed on May 27, 2016 which is hereby incorporated by reference herein in its entirety.
The present invention relates to a magnetic rotation detection apparatus that detects an absolute rotation angle, and particularly to a magnetic rotation detection apparatus for detection of a gear position of a motorcycle.
Description of the Related Art
A magnetic rotation detection apparatus (gear position sensor) that detects a gear position selected by a driver, such as first position or second position, in a motorcycle has been known. The gear position sensor is connected to a shift drum which is built in transmission and detects the gear position based on a rotation angle of the shift drum.
Conventionally, a gear position sensor 120 includes a shaft 121 that rotates in response to rotation of the shift drum and a circuit board 122 that is arranged such as to face the shaft 121 on the extension of a rotation axis of the shaft 121 as illustrated in FIG. 13. A magnet 123 serving as a permanent magnet is provided at an end facing the circuit board 122 of the shaft 121, and a Hall element 124 is provided at the circuit board 122 such that the rotation angle of the shift drum is detected as the Hall element 124 detects a change of magnetic field intensity accompanying rotation of the magnet 123 which rotates along with the shaft 121 (for example, see Japanese Patent No. 5148418).
In the gear position sensor 120, gaps are positively provided among the respective constituent elements and the shaft 121 in order to implement smooth rotation of the shaft 121. Accordingly, the magnet 123 of the shaft 121 and the circuit board 122 do not contact each other, and a gap is provided between the magnet 123 and the circuit board 122 in a rotation-axis direction (hereinafter, referred to as a “thrust direction”) of the shaft 121.
Recently, output of an engine should be controlled precisely, and the gear position is considered at the time of executing traction control of the engine output. In addition, stages of a transmission have been also increased in order for improvement of fuel economy and improvement of drivability. As a result, there is a demand for gear position detection with higher accuracy. Meanwhile, the magnetic field intensity is changed due to movement of the thrust direction of the shaft 121 (the magnet 123) as well as the rotation of the shaft 121 since the gap is provided between the magnet 123 and the circuit board 122 in the thrust direction in the conventional gear position sensor 120 as described above, which makes it difficult to detect the gear position with higher accuracy.
To cope with this, the present inventors and his/her colleagues have studied a method of holding the shaft 121 with an oil seal 125 as illustrated in FIG. 14. The oil seal 125 is in contact with the shaft 121 with high surface pressure, and thus, can suppress movement of the shaft 121 (the magnet 123) in the thrust direction.
Meanwhile, the shaft 121 and the circuit board 122 are housed inside a case 126 of the gear position sensor 120. As described above, the shaft 121 is held by the oil seal 125, and the circuit board 122 is held by a holder 127 while the holder 127 abutting on an inner wall of the case 126. Further, a potting resin 128, such as resin, is injected into a space formed by the circuit board 122 and the inner wall of the case 126, and a position of the circuit board 122 inside the case 126 is fixed as the potting resin 128 is cured. In order to prevent the potting resin 128 from flowing into the gap between the circuit board 122 and the magnet 123 through a gap between the inner wall of the case 126 and a wall portion of the holder 127 that opposes the inner wall of the case 126 (hereinafter, referred to as an “facing wall”) at the time of injecting the potting resin 128, an O-ring 129 is provided at the facing wall of the holder 127 in the gear position sensor 120, and the O-ring 129 seals the gap between the inner wall of the case 126 and the facing wall of the holder 127.
However, the shaft 121 has been already arranged inside the case 126 and held by the oil seal 125 when the holder 127 is inserted inside the case 126, and the oil seal 125 does not leak air at the time of moving the holder 127 toward the shaft 121 (the magnet 123) inside the case 126 so that air inside a space 130 surrounded by the O-ring 129 of the holder 127 and the oil seal 125 is compressed, and internal pressure of the space 130 increases. As a result, high-pressure air from the space 130 shifts the O-ring 129 from the facing wall of the holder 127, and the O-ring 129 fails to seal the gap between the inner wall of the case 126 and the facing wall of the holder 127 in some cases. That is, there is a risk that assembly failure of components of the gear position sensor 120 may occur. When the assembly failure of components of the gear position sensor 120 occurs, the potting resin 128 flows into the gap between the circuit board 122 and the magnet 123, and there is a risk that the accuracy in detection of the change of the magnetic field intensity accompanying the rotation of the magnet 123, that is, the rotation angle of the magnet 123 using the Hall element 124 may decrease.